Resonant and semi-resonant DC-DC converters, including isolated and non-isolated topologies, are used in a variety of applications including telecommunications, consumer electronics, computer power supplies, etc. The usage of such converters is gaining popularity because of their zero-voltage switching (ZVS) and/or zero-current switching (ZCS) characteristics, and their ability to utilize parasitic electrical properties inherent in an electronic circuit. Among numerous topologies, the semi-resonant converter with transformer/tapped inductor is an attractive topology for providing high voltage-conversion ratios without requiring isolation. Such converters provide advantages including lower cost and higher efficiency as compared to other solutions.
One class of semi-resonant converters includes high-side and low-side switches that transfer power from an input source to a tapped inductor that supplies output power to a load. The tapped inductor is also connected to a second low-side switch, which is termed a synchronous rectification (SR) switch herein. In order to meet the power requirements for a load of a semi-resonant converter (e.g., provide a near constant output voltage for the load), many semi-resonant DC-DC converters employ a variable switching frequency wherein the switching period can vary from cycle to cycle. During a portion of each switching period, the SR switch will be enabled such that current flows through it. For the semi-resonant converter described above, the current during this portion of a switching period will be shaped like one half cycle of a sinusoidal period. The time interval for this half-cycle sinusoid is determined by reactive elements within passive circuitry of the semi-resonant converter, e.g., the natural frequency of an inductor/capacitor (LC) resonant tank and other passive components within the semi-resonant DC-DC converter determine this time interval. As such, each phase has its own natural frequency.
It is highly desirable to turn the power switches of a resonant or semi-resonant DC-DC converter on and off when the voltage or current across the relevant switch is at or near zero. Such soft switching has an advantage that switch losses are minimized. Additionally, soft switching avoids electromagnetic interference (EMI) that is due to high-frequency harmonics associated with hard switching. An important consequence of these advantages is that soft-switching resonant and semi-resonant converters can run at much higher efficiencies than their corresponding hard-switching counterparts. However, the transfer function can change under different conditions such as changing load, input voltage, output voltage, etc. which can result in non-linear behaviour.
Accordingly, there is a need for improved control adaptation techniques which mitigate non-linear behaviour in resonant or semi-resonant DC-DC converter that use synchronous rectification (SR) switches.